Cigarette smoking is linked to numerous health problems. But now, researchers say that smoking can also increase the risk of venous thromboembolism, according to a study published in the journal PLOS Medicine.

Venous thromboembolism (VTE) is the third most common cardiovascular illness. It is a combination of deep vein thrombosis (blood clots in the legs) and pulmonary embolism (blood clots in the lungs).

According to the Centers for Disease Control and Prevention (CDC), between 2007 and 2009 in the US, an average of 547,596 adults were hospitalized each year as a result of VTE.

For the study, researchers from Sun Yat-Sen University in Guangzhou, China, conducted a review of 32 observational studies involving 3.9 million participants and 35,151 VTE “events.”

The findings showed that participants who were current smokers had a 23% increased risk of VTE, compared with those who had never smoked.

Those who smoked in the past and present showed a 17% increased risk of VTE, compared with those who had never smoked, while those who formerly smoked showed a 10% increased risk.

Additionally, the study revealed that the more pack-years (the number of cigarette packs smoked a day, multiplied by the number of years a person has smoked) and daily cigarettes a person smoked, the higher the risk of VTE.

Every additional 10 cigarettes per day increased the risk by 10.2%. For every additional 10 pack-years, the risk of VTE was increased by 6.1%.

The study authors say:

This meta-analysis is the first to our knowledge to confirm smoking to be an independent risk factor for VTE. Our results suggest that the association of smoking with VTE risk may be largely mediated by an acute mechanism, supported by a dose-response relationship for the amount of current smoking and the higher risk in current compared to former smokers.”

“In addition,” they continue, “the association was not solely due to secondary smoking-related diseases, because we found a positive association between current smoking and both unprovoked and provoked VTE.”

“However, a relatively weak association between former smoking and the risk of VTE was also observed. We suppose this association may be mediated by secondary smoking-related diseases.”

When adjusting the results for body mass index (BMI) of the participants, the researchers found that the risk of VTE increased to 30%. The researchers say that this increase may be because high BMI is also a risk factor linked to VTE, while smoking is linked to a lower BMI.

The researchers conclude that the link between smoking and VTE has “clinical relevance” when it comes to individual screening, modifying current risk factors, and determining primary and secondary prevention of the disorder.

They add that further studies are needed in order to better understand the underlying processes that link smoking to increased risk of VTE.